Letter pubs.acs.org/Langmuir
Acoustic Tracking of Cassie to Wenzel Wetting Transitions Renaud Dufour,† Nadine Saad,‡,§ Julien Carlier,§ Pierre Campistron,§ George Nassar,§ Malika Toubal,§ Rabah Boukherroub,∥ Vincent Senez,† Bertrand Nongaillard,§ and Vincent Thomy*,† †
Institute of Electronics, Microelectronics and Nanotechnology (IEMN, UMR 8520) Cité Scientifique, University of Lille Nord de France, Avenue Poincaré, BP 60069, 59652 Villeneuve d’Ascq, France ‡ LPA, EDST, Université Libanaise, P.O. Box 90656, Jdeidet, Lebanon § Institute of Electronics, Microelectronics and Nanotechnology (IEMN, UMR 8520), Université de Valenciennes et du Hainaut-Cambrésis, Le Mont Houy 59313, France ∥ Institut de Recherche Interdisciplinaire (IRI, USR 3078), Université Lille 1, Parc de la Haute Borne, 50 Avenue de Halley, BP 70478, 59658 Villeneuve d’Ascq, France ABSTRACT: Many applications involving superhydrophobic materials require accurate control and monitoring of wetting states and wetting transitions. Such monitoring is usually done by optical methods, which are neither versatile nor integrable. This letter presents an alternative approach based on acoustic measurements. An acoustic transducer is integrated on the back side of a superhydrophobic silicon surface on which water droplets are deposited. By analyzing the reflection of longitudinal acoustic waves at the composite liquid−solid− vapor interface, we show that it is possible to track the local evolution of the Cassie-to-Wenzel wetting transition efficiently, as induced by evaporation or the electrowetting actuation of droplets.
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INTRODUCTION Over the past decade, liquid-repellent materials have been the subject of intensive investigations. Superhydrophobic and superomniphobic surfaces have been developed using many different approaches,1−3 and wetting phenomena have been extensively studied both experimentally and theoretically through measurements of the contact angle, contact angle hysteresis,4−8 wetting transition mechanisms,9,10 and drag reduction properties.11,12 More recently, these studies have led to the emergence of a number of innovative and promising applications in various fields such as biosensors,13 cell biology,14,15 chemistry,16 and self-assembly17,18 and even droplet logic, data storage, and display.19,20 These applications take advantage of the different wetting states that can be achieved on superhydrophobic surfaces.21 Considering a surface made of micrometer-scaled structures as shown in Figure 1, a liquid can either sit on top of the features in a so-called Cassie wetting state or spread between the asperities (Wenzel state). The liquid can also adopt an intermediate configuration in which the roughness is partially filled. In an applied prospective and for most of the aforementioned applications, it becomes more and more important to develop strategies allowing to (i) actively switch between these different superhydrophobic states and (ii) monitor in real time the wetting state of a liquid. The first point has been actively investigated in the last few years, for instance, by using mechanical, magnetic, or electric actuation to control wetting transitions.19,22,23 However, the number of monitoring methods is still limited. © 2013 American Chemical Society
Figure 1. Different wetting configurations of a water droplet deposited on a superhydrophobic surface.
To date, wetting states and wetting transitions are commonly monitored on a mesoscopic level by measuring the droplet contact angle θ or contact angle hysteresis or visualizing the air pockets trapped in the asperities using optical methods.24 However, in some cases, the different wetting configurations can lead to similar values of θ and/or similar hysteresis or it can be difficult to distinguish the air pockets (particularly for submicrometer roughness). To overcome these limitations, Received: July 1, 2013 Revised: September 28, 2013 Published: October 11, 2013 13129
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more accurate optical techniques have been developed using either interference contrast microscopy, which enables the reconstruction of the liquid−gas interface below the drop,25,26 confocal microscopy,27 or laser beam reflection.28 These approaches are necessary to gain deeper insight into the underlying physical mechanisms of wetting transitions, revealing, for instance, the shape of liquid menisci between adjacent microstructures and the formation of parabolic penetration profiles for evaporating drops.25,26 However, in an applied prospective, confocal or interference microscopy methods as well as laser beam reflection techniques are limited because they require either specific setups, transparent materials, or the addition of fluorescent dyes to the liquid. Consequently, they are limited in term of integration and versatility for in situ monitoring of wetting states in microfluidic applications. Among the nonoptical techniques, Tuberquia and co-workers demonstrated the use of electrochemical impedance spectroscopy to capture the Cassie-to-Wenzel transition on polymethylene superhydrophobic surfaces.29 Other approaches rely on quartz crystal resonators30 or longitudinal acoustic wave reflection31 to measure the degree of liquid penetration between microstructures or nanostructures. In particular, in a recent study we demonstrated that the reflection of longitudinal acoustic waves could differentiate both Cassie and Wenzel wetting states (addressed individually) for droplets at rest on superhydrophobic silicon micropillar arrays.31 The droplets were placed opposite acoustic transducers fabricated directly on the rear side of the superhydrophobic surface. Contrary to the aforementioned advanced microscopy techniques, this acoustic method does not capture the liquid−vapor meniscus but is based on measurements at the solid−vapor and solid−liquid interfaces to describe the average wetting state of a sample. Furthermore, the acoustic method presents a strong potential in terms of integration and versatility due to the transducer position on the back side of the superhydrophobic surface, without access constraint on the surface under investigation. In this letter, acoustic transducers are used to track the localized evolution of the wetting transition from the Cassie to Wenzel state. Transitions are induced by two methods, which are (i) droplet evaporation32,33 and (ii) electrowetting actuation.34−36
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Figure 2. (a) Scanning electron microscopy image of the silicon micropillars array. (b) Pillar dimensions are diameter a = 15 μm, pitch b = 30 μm, and height h = 20 μm, and the thickness of bulk silicon is eSi ≈ 400 μm. A circular acoustic transducer (250 μm in diameter) is patterned on the rear side of the substrate. The acoustic reflection coefficients measured at the top and bottom interfaces of the micropillars with the liquid are respectively r*top and r*bottom.
Table 1. Normalized Acoustic Reflection Coefficients at the Top and Bottom Part of a Micropillars Array in Contact with Air (Used as Reference) and Contacting a Liquid in the Cassie or Wenzel State * rtop r*bottom
air
Cassie
Wenzel
1.00 1.00
0.86 1.00
0.75 0.86
covered by water in a homogeneous Cassie or Wenzel state). As far as these coefficients are concerned, they depend on the materials involved, the microstructural dimensions, and the wetting configuration. For a solid−air interface, the reflection coefficient equals 1. When liquid contacts the silicon surface, the coefficient drops to 0.86 (top reflection for the Cassie state or bottom reflection for the Wenzel state). A particular case is for the top reflection in the Wenzel state, where r*top is further decreased to 0.75 because of water surrounding the pillar. This phenomena was discussed in detail previously.31 As will be discussed further, if the transducer is partially covered by the droplet or if an intermediate wetting configuration is achieved, then the coefficients will present intermediate values. It is to be noted that the amplitude of the electrical signal used for transducer excitation is lower than 10 dBm, which is not sufficient to modify the nature of the interface and the state of the droplet. The transmitted acoustic power is lower than 1 mW, and the power required to promote liquid penetration in porous materials is typically in the range of 1 W to tens of watts.38−40
EXPERIMENTAL SECTION
The superhydrophobic surfaces are made of silicon micropillars (dimensions given in Figure 2) coated with a plasma-deposited fluoropolymer (C4F8). Advancing and receding contact angles of water on a flat silicon surface coated with fluoropolymer are respectively 98 ± 2 and 122 ±2°. Acoustic transducers are fabricated on the rear face of the substrate and they consist of the following stack: Pt (100 nm)/ZnO (2.4 μm)/Pt (100 nm)/Ti (10 nm)/Si. The transducers are 250 μm in diameter and generate 800 MHz longitudinal acoustic waves. Details concerning the fabrication processes were reported previously.31,37 The acoustic measurement is based on echography: a longitudinal acoustic wave propagates through the substrate and is reflected at the surface. The high-frequency bandwidth of the transducers makes it possible, in the time domain, to separate the echoes rising from the top and bottom parts of the micropillars. The reflection coefficients are respectively r*top and r*bottom (Figure 2b). They are obtained by analyzing the reflected acoustic wave amplitude and normalized with respect to a calibration measurement in air. Further details regarding the method and calculation are given in refs 31 and 37. Table 1 summarizes the reflection coefficients expected for pure Cassie and pure Wenzel wetting states31 (i.e., the whole surface in front of the transducer is
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RESULTS AND DISCUSSION A wetting transition was first initiated by evaporating a 15 μL water drop initially sitting on top of the pillars in a Cassie wetting state, with the acoustic transducer opposing the drop center. In that case, the drop shrinks and increases its internal Laplace pressure until a spontaneous transition occurs.32 13130
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combination of a relatively high receding contact angle along with a decreasing base diameter33). At t ≈ 11 min, a transition occurs: θ decreases to ∼70° and the base diameter becomes constant as a result of strong pining in the Wenzel state. From this time, no reliable optical measurement can be made, and a water film is observed to vanish in a few seconds. Acoustic results are displayed in Figure 3b. The first measurement is performed prior to drop deposition (at t = 0), in which rtop * and rbottom * are equal to 1 (acoustic wave reflection occurs at the silicon−air interface only). When the drop is deposited (at t ≈ 2 min), r*top drops from 1 to 0.86 because of the change in acoustic impedance at the top of the pillars. For 2 < t < 11 min, evaporation occurs. Because the drop remains in a Cassie wetting state and its footprint is larger than the transducer, no notable change is observed for the reflection coefficients. A spontaneous wetting transition is observed after t = 11 min. rtop * and rbottom * respectively reach values of 0.75 and 0.86, corresponding to the Wenzel wetting state (Table 1). Finally, when the drop is no longer observable, at t = 13 min rtop * and rbottom * recover almost their initial value of 1, with the slight difference attributed to the presence of a thin water film. From these acoustic measurements, the values obtained for r*top and r*bottom at the Wenzel transition match the ones obtained in static mode. We can thus assess that the imbibition of the droplet happened exactly above the acoustic transducer (100% of its surface was covered with wetted pillars). This point has been confirmed by the superposition of images taken on the back side and front side. For a second time, we used the acoustic transducer to track a wetting transition triggered by electrowetting on a dielectric (EWOD).34 EWOD is a common and versatile approach to modulating drop spreading in digital or continuous microfluidic devices. It consists of applying an electric field between a liquid and a conductive surface separated by a dielectric layer. For EWOD actuation, a SiO2 layer (300 nm thick) was grown on the pillars by chemical vapor deposition (CVD) to act as a dielectric (relative permittivity εr = 3.9). The surface was then hydrophobized with a plasma-deposited fluoropolymer (C4F8).31 EWOD actuation on a superhydrophobic surface typically results in a two-step process:36 (i) When increasing the applied voltage, the drop first spreads on the surface because of an increase in the Maxwell stress in the vicinity of the contact line (Figure 4a), resulting in a so-called 1D transition: only the asperities adjacent to the droplet rim are filled, which makes the droplet optically seem to be in a Wenzel configuration, although air-pocket trapping (i.e., a Cassie state) is still achieved below the drop.42 (ii) Above a threshold voltage U*, the Maxwell stress overcomes the maximal capillary pressure sustainable by the pillars. As a consequence, a Cassie-to-Wenzel transition is initiated at the contact line (where the electric field is maximized) and propagates below the drop, back to its center (Figure 4a) (i.e., the 1D transition propagates to a 2D transition). For the EWOD experiment, a 10 μL water drop is deposited on the surface in an initial Cassie configuration. The transducer is placed opposite the drop periphery in order to track the imbibition phenomena (Figure 4b). Voltage Urms is increased from 0 to 250 V with a step of 25 V. For each voltage, rtop * and r*bottom are measured, and the results are displayed in Figure 4c. The transition from Cassie to Wenzel occurs here for a * ≈1 threshold voltage of Urms* ≈ 75 V. Below this value, rbottom because the liquid does not wet the bottom surface (air totally
During the evaporation process, optical characterization was performed (measurement of the contact angle θ and drop base diameter d), along with a measurement of reflection coefficients rtop * and rbottom * . Because of limited optical access to the setup, the droplet was observed with an angle of about 30°, leading to a systematic underestimation of the contact angle.41 Optical and acoustic results, obtained simultaneously on our homemade setup, are displayed in Figure 3. Measurements made on
Figure 3. Results obtained for a 15 μL water droplet evaporated on the superhydrophobic micropillar array. (a) Optical measurement: contact angle θ and normalized base diameter d/d0. (b) Acoustic measurement: evolution of reflection coefficients on the top and bottom parts * and rbottom * , respectively. At t = 0, an acoustic of the pillars, rtop measurement is performed without liquid, so r*bottom and r*bottom equal 1. The drop is deposited at t = 2 min.
a classical goniometer (DSA 100, Kruss, Germany) demonstrated that our microstructured surface sustains a Cassie state all along the evaporation process (down to a droplet base diameter of about 200 μm). At t = 2 min, the contact angle equals 122 ± 2°. Up to t = 9 min, the drop evaporates in a constant base diameter mode because of contact line pining by the pillars32,33 (θ decreases and d remains constant). From t = 9 to 11 min, a slight decrease in the contact angle is observed (θ remains around a receding value of θr ≈ 100°), and the drop base diameter decreases. Up to this moment, a Cassie state is maintained (i.e., 13131
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of around 500 μm, the transducer diameter has to be larger than 130 μm to be in the near field and avoid beam diffraction.
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CONCLUSIONS AND OUTLOOK The experiments presented in this letter demonstrate that sound wave compression is a sensitive method for monitoring the evolution of the Cassie to Wenzel transition on microstructured superhydrophobic surfaces. Evaporation and electrowetting methods were used to trigger different types of Cassie-to-Wenzel transitions, and we observed that both the drop-wetting configuration and transition thresholds are clearly distinguished when we look at the acoustic reflection coefficients. Furthermore, measurements have shown that composite wetting states (for which liquid partially penetrates the pillars lattice) can be monitored. Using this method, it is possible to determine the wetting state under the droplet without any direct access to the droplet, which is not possible with some of the recent optical characterization. This approach appears to be an interesting alternative to the usual optical measurement. Its integration can be considered within microfluidic devices, and its sensitivity could allow the wetting characterization of other kinds of surfaces, presenting nanoscale structures or a random roughness,43 along with surfaces made of different materials such as polymers.44
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AUTHOR INFORMATION
Author Contributions
The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The Lebanese part of this study was supported by the Lebanese CNRS. The French part of the research was supported by the French CNRS. This work was supported by the region NordPas-de-Calais and FEDER through the CPER-CIA project, by the French government program “Investissements d’Avenir” managed by the National Research Agency (ANR) under contract ANR-11-EQUIPEX-0025, and by the Ministry of Defense.
Figure 4. Monitoring of EWOD-induced liquid imbibition. The drop is initially in a Cassie wetting state. As the applied voltage is increased from 0 to 250 V, water first spreads until a wetting transition is initiated at the contact line. (a, b) Schemes of the experimental setup (position of the liquid/substrate interface compared to the transducers). The acoustic transducer is placed opposite the drop periphery. (c) Evolution of acoustic reflection coefficients with applied voltage. The beginning of the filling transition is indicated by the vertical dashed line, corresponding to a simultaneous decrease in the two coefficients.
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reflects the acoustic wave). rtop * has an initial value of ∼0.97, meaning that at Urms = 0 most of the pillars are exposed to air. As U is increased from 0 to 75 V, r*top slightly decreases from 0.97 to 0.94 because of drop spreading. (We can roughly estimate that the fraction of wetted pillars in front of the transducer increases from 20 to 40%). Above Urms*, both coefficients sharply decrease, finally reaching values of 0.86 and 0.75 corresponding to the final Wenzel wetting state. This result demonstrates that the acoustic method is efficient to track the transition propagation below the drop and thereby to distinguish between 1D and 2D types of transitions. As a perspective to go further in understanding the dimensions of the transition, it would be possible to reduce the size of the transducer in order to get more local information (thereby increasing the spatial resolution). The transducer size is limited by the acoustic wavelength (around 8 μm in silicon for 1 GHz longitudinal acoustic waves) and beam diffraction from the transducer to the interface. For a typical silicon wafer thickness
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consequence, it would be possible to perform such measurements on polymer-based superhydrophobic surfaces.
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